Driving on a monotonous road such as an expressway easily makes the driver drowsy. In other words, a decrease in wakefulness (arousal level) of the driver easily occurs.
As a technique of suppressing such decrease in wakefulness of a driver, a method including changing music characteristics at particular intervals has been proposed (see, for example, PTL 1). In PTL 1, one of sound characteristics that stimulate senses of a person is changed at regular or random intervals that are equal to or shorter than a half cycle of his/her sleep-wake rhythm to maintain wakefulness of a driver.